The clinical circumstances of the preterm and sick term infant are quite complex from a metabolic and nutritional viewpoint. The calculation of protein catabolism and synthesis in infants with a wide variety of disease states could be used as a guideline to help recognize the infants who are at a particular nutritional risk and identify those who are becoming increasingly catabolic. The information gained from the quantification of protein breakdown in neonates and the examination of net protein synthesis vs. catabolism, would provide a more accurate estimate of protein requirements by neonates and potentially maximize their nutritional status.